Heat exchanger

ABSTRACT

A new aluminium alloy containing (in wt. %): 0.2-0.5 Fe; 0.7-1.2 Si; 1.2-1.6 Mn; up to 0.3 Mg; up to 0.5 Cu; up to 0.2 Zn, up to 0.1 Ti is used to make the fins of heat exchangers particularly car radiators. The finstock has high post braze strength and thermal conductivity, and has a sufficiently electronegative potential as to be capable of acting as a sacrificial anode for the heat exchanger tubes. By virtue of the absence of Sn, In and Cr, these heat exchangers can be scrapped and melted for re-use.

[0001] Aluminium alloy heat exchangers, provided with header plates,tank units, tubes for coolant (water) and fins for improved heatexchange, are very widely used in the automobile industry and elsewhere.Generally the fins are joined to the tubes by brazing e.g. by theNOCOLOK™ technique or under vacuum. In order to reduce corrosion of thetubes with consequent leakage of coolant, it has been common practice tomake the fins electronegative relative to the tubes, so that the finsact as sacrificial anodes. This may be done by adding Zn, Sn, or In tothe metal used to form the fins.

[0002] After prolonged use, discarded heat exchangers are routinelyrecovered and re-melted together with process scrap for re-use. But thepresence of Zn, Sn or In in tube, header plate or tank units wouldsignificantly reduce their corrosion resistance. So the melt can only beused in the production of casting alloy of rather wide compositionranges and correspondingly low value. Similarly Cr is undesirable in Alalloy metal to be melted for re-use. There is a need in the automotiveindustry to provide heat exchangers that can be recycled back into heatexchanger components. It is one object of this invention to meet thatneed.

[0003] In one aspect the invention provides an alloy having thecomposition. Component Broad (wt %) Preferred Fe 0.2-0.5 0.3-0.5 Si0.7-1.2 0.8-1.0 Mn 1.2-1.6 1.25-1.5 Mg up to 0.3 0.07-0.13 Cu up to 0.50.27-0.4 Zn up to 0.2 up to 0.1 Ti up to 0.1 up to 0.05 Others up to0.05 each, 0.15 total Al Balance

[0004] Although this alloy can be used for tube plate, side supports andheader tanks in heat exchanger units, and may have other uses, it isprimarily intended as a finstock alloy for heat exchangers. The alloycan be stronger, so the finstock can be thinner and lighter and havebetter thermal conductivity than conventional finstock alloys.

[0005] The heat exchanger market, particularly in the automobileindustry, requires that finstock alloys offer a balance of properties,i.e. strength, formability, corrosion resistance and brazability. A keyfeature of the novel alloys of this invention is the relatively high Sicontent. This increases the post-braze strength, by more than 10%relative to conventional finstock alloys, and also improves thepost-braze thermal conductivity. Also, when fins of this alloycomposition are joined to tubes by brazing, the Si-rich solid solutionthat results makes the fin more electronegative and ensures that it actsas a sacrificial anode for the tube. Si also reduces the melting pointof the alloy. An upper limit on the Si content is set by the need forthe alloy to have a sufficiently high solidus to be brazable.

[0006] Mn is a strengthening component and enough should be used toallow this effect to be seen. However, if too much is used, then coarseMn−Fe intermetallics may be used which reduce formability and mechanicalproperties. Preferably the (Mn+Fe) content is no more than 1.9% or even1.8%.

[0007] Fe is generally present in secondary Al alloys. In this case, itscontent needs to be controlled to permit high levels of Mn to bepresent. Alloys containing very low levels of Fe are less attractiveowing to the increased cost of exercising tight control over the Fecontent.

[0008] Mg is preferably present as a strengthening component. At highconcentrations an undesired MgO deposit is formed on the metal surfaceduring brazing. The Mg concentration is controlled at levels where thisis not a problem.

[0009] Cu is preferably included as a strengthening component. Cu isbelieved not to reduce corrosion resistance in a way that has previouslybeen reported. Alloys containing high concentrations of copper aredifficult to cast, but if these problems can be overcome then Cuconcentrations up to 0.5% or even 1.0% are useful.

[0010] Zn makes the alloy more electronegative, so the Zn content shouldbe at a level below that which results in rapid corrosion. Preferably Znis not deliberately added.

[0011] Ti is used as a grain refiner, typically at a concentration ofabout 0.02%.

[0012] Other components including Sn, In and Cr are not deliberatelyadded to the alloy, but may be present as impurities in concentrationsup to 0.05% each, 0.15% total. The balance of the alloy consists of Al.

[0013] In another aspect, this invention provides a brazed heatexchanger having fins of the alloy defined. Preferably the brazed heatexchanger has tubes (and optionally also tanks, header plates, etc.), ofan alloy having the composition. Component Wt % Mn 0.7-1.5 Cu 0.1-1.0 Feup to 0.4 Si up to 0.2 Mg up to 0.8 V and/or Cr up to 0.3 Zn up to 0.2preferably up to 0.1 Ti up to 0.1 Others up to 0.05 each, 0.15 total Albalance

[0014] These alloys are described in U.S. Pat. Nos. 5,037,707 and5,041,343, and in WO 94/22633, all in the name of Alcan InternationalLimited. The corrosion resistant properties of these alloys are enhancedby the relatively high Mn and particularly Cu contents, and byexercising strict control over the levels of Fe, Zn and particularly Si.Heat exchangers with tubes of these published alloys containing 0.5-1.0%Cu, and with fins of the above finstock alloy have excellent corrosionresistance allied to good formability, brazability and post-brazestrength. With a reduced rate of corrosion attack on the fin, heattransfer performance will be maintained. The additional properties ofhigher post-brazed strength and increased sag resistance offer theopportunity for down gauging and reducing jigging of units prior to thebrazing cycle.

[0015] Clad brazing sheet is often used to make brazed heat exchangertubes, tanks, header plates, etc. The clad brazing sheet has a corewhose composition is not material but may be as shown above; andcladding on one or both surfaces of an alloy that has a low meltingpoint by virtue of containing a high concentration of Si. Process scrapfrom such clad brazing sheet can be remelted and used to make finstockor other heat exchanger components.

[0016] In yet another aspect, the invention provides a method of makingheat exchanger components, by forming an Al alloy melt comprisingdiscarded heat exchangers and/or clad brazing sheet scrap, adjusting thecomposition of the melt, and using the resulting melt to make heatexchanger components, characterised in that discarded heat exchangersare brazed heat exchangers as defined above.

[0017] In this practice, an aluminium alloy melt is formed usingdiscarded heat exchangers and/or brazing sheet scrap and possibly otherscrap. The composition of the melt is adjusted, by the addition of Almetal or other components as required. Then the melt is cast intoingots, which are rolled or extruded by conventional methods to providethe required heat exchanger components. Standard recycling techniquescan cope quite adequately with raised Si levels, such as may result fromthe use of discarded brazed heat exchangers according to this inventionand/or clad brazing sheet scrap.

EXAMPLE 1

[0018] A 7000 kg ingot was DC cast using conventional techniquepreheated to 520° C. and hot rolled to a thickness of 3.5 mm. The sheetwas then cold rolled to 0.4 mm and annealed at 360° C. for 2 hoursbefore further cold rolling to 0.110 mm. This is the H18 temper.

[0019] The ingot had the composition: 0.4% Fe; 0.9% Si; 1.3% Mn; 0.1%Mg; 0.3% Cu; balance Al of commercial purity.

[0020] The cold rolled sheet had the following mechanical and electrochemical properties after being subjected to a commercial brazing cycle(approximately 5 minutes at 605° C. and air cool).

[0021] AA3003 is a commercial alloy used for finstock and subjected tothe same brazing cycle. This Invention AA3003 UTS 162 MPa 135 MPa Proof 59 MPa  40 MPa Ductility 18% 20% Corrosion potential (ASTM G69) −750 to−780 mV −690 to −710 mV

[0022] The corrosion potential is sufficiently negative to make themetal useful as sacrificial anodes in brazed heat exchangers. Withoutthe addition of a substantial proportion of Zn, the AA3003 alloy wouldnot have been useful in this sense.

[0023] Another ingot of the same invention composition was hot rolled to3 mm, cold rolled to 0.18 mm, interannealed at 360-400° C. for 2 hours,and finally cold rolled to 0.110 mm. This is the H14 temper. Thepost-braze properties would have been essentially the same as those setout above.

EXAMPLE 2

[0024] The following alloys were investigated. Component (wt %) X800X900 3003 Invention Fe <0.4 <0.4 <0.7 0.4 Si  <0.15  <0.15 <0.6 0.9 Mn0.7-1.5 0.7-1.5 1.0-1.5 1.3 Mg <0.8 <0.8 — 0.1 Cu 0.1-0.6 0.5-1.00.05-0.20 0.3 Zn — — <0.1 — Ti <0.1 <0.1 — — V/Cr <0.3 <0.3 — —

[0025] The thermal conductivity of AA3003 in the 0 temper is 185 W/mK.The thermal conductivity of the invention alloy is 215 W/mK.

[0026] The galvanic ranking of these alloys, as determined by ASTM G69(1994), in a post-braze state is as follows:

[0027] X800 is −715 mV

[0028] X900 is −730 mV

[0029] 3003 is −730 mV

[0030] Invention alloy is −740 mV.

[0031] The differences between X800 and X900 on the one hand and theinvention alloy on the other hand are in the range 10-25 mV. Thesedifferences are sufficient to allow fins of the invention alloy to actas sacrificial anodes; but not so great as to encourage rapidsacrificial corrosion. This was illustrated by means of a ZRA galvaniccompatibility experiment, in which the invention alloy was used forfinstock and the X900 alloy for tubestock. The difference in corrosionpotentials (ASTMG 69) was 15 mV. The following values were obtained inthe ZRA experiment:

[0032] Current-8 μA

[0033] Corrosion rate-90 μm/yr.

1. An alloy having the composition Component Broad (wt %) Preferred Fe0.2-0.5 0.3-0.5 Si 0.7-1.2 0.8-1.0 Mn 1.2-1.6 1.25-1.5 Mg up to 0.30.07-0.13 Cu up to 0.5 0.27-0.4 Zn up to 0.2 up to 0.1 Ti up to 0.1 upto 0.05 Others up to 0.05 each, 0.15 total Al Balance


2. A brazed heat exchanger having fins of the alloy of claim 1 .
 3. Abrazed heat exchanger according to claim 2 , having tubes of an alloyhaving the composition Component Wt % Mn 0.7-1.5 Cu 0.1-1.0 Fe up to 0.4Si up to 0.2 Mg upto 0.8 V and/or Cr up to 0.3 Zn up to 0.2 preferablyup to 0.1 Ti up to 0.1 Others up to 0.05 each, 0.15 total Al balance


4. A method of making heat exchanger components, by forming an Al alloymelt comprising discarded heat exchangers and/or clad brazing sheetscrap, adjusting the composition of the melt, and using the resultingmelt to make heat exchanger components, characterised in that discardedheat exchangers are brazed heat exchangers according to claims 2 orclaim 3 .